Fuel cells provide electricity from chemical oxidation-reduction reactions and possess significant advantages over other forms of power generation in terms of cleanliness and efficiency. Typically, fuel cells employ hydrogen as the fuel and oxygen as the oxidizing agent. The power generation is generally proportional to the consumption rate of the reactants.
A significant disadvantage which inhibits the wider use of fuel cells is the lack of a widespread hydrogen infrastructure. Hydrogen has a relatively low volumetric efficiency and is more difficult to store and transport than the hydrocarbon fuels currently used in most power generation systems. One way to overcome this difficulty is the use of reformers to convert the hydrocarbons to a hydrogen-rich gas stream that can be used as a feed for fuel cells.
Fuel reforming processes, such as steam reforming, partial oxidation, and autothermal reforming, can be used to convert hydrocarbon fuels such as natural gas, LPG, gasoline, and diesel, into a hydrogen rich gas. In addition to the desired product hydrogen, undesirable byproduct compounds such as carbon dioxide and carbon monoxide are found in the product gas. For many uses, such as fuel for proton exchange membrane (PEM) or alkaline fuel cells, these contaminants reduce the value of the product gas.
In a conventional steam reforming process, a hydrocarbon feed, such as methane, natural gas, propane, gasoline, naphtha, or diesel, is vaporized, mixed with steam, and passed over a steam reforming catalyst. The majority of the feed hydrocarbon is converted to a mixture of hydrogen, carbon monoxide, and carbon dioxide. The reforming product gas is typically fed to a water-gas shift bed in which much of the carbon monoxide is reacted with steam to form carbon dioxide and hydrogen. After the shift step, additional purification steps are needed to bring the hydrogen purity to the desired level. These steps include, but are not limited to, selective oxidation to remove remaining carbon monoxide, flow through a hydrogen permeable membrane, and pressure swing absorption.
For use in a PEM fuel cell the reformate hydrogen purity that is specified can vary widely between 35% and 99.999% with very low ( less than 50 ppm) carbon monoxide level desirable. Generally, higher hydrogen purity improves fuel cell efficiency and cost. For alkaline fuel cells, low carbon dioxide levels are needed to prevent formation of carbonate salts. For these and other applications, an improved steam reforming process capable of providing a high hydrogen, low carbon monoxide, low carbon dioxide reformate is greatly desired.
The present disclosure is generally directed to a method for converting hydrocarbon fuel to hydrogen rich gas. In one such illustrative embodiment, the method includes: reacting the hydrocarbon fuel with steam in the presence of reforming catalyst and a carbon dioxide fixing material to produce a first hydrogen gas; and removing carbon monoxide from the first hydrogen gas to produce the hydrogen rich gas. The carbon monoxide removing step utilizes either methanation or selective oxidation. The carbon dioxide fixing material is preferably selected so as to substantially reduce the content of the carbon dioxide present in the hydrogen containing gas. Illustrative materials include calcium oxide, calcium hydroxide, strontium oxide, strontium hydroxide, or minerals such as allanite, andralite, ankerite, anorthite, aragoniter, calcite, dolomite, clinozoisite, huntite, hydrotalcite, lawsonite, meionite, strontianite, vaterite, jutnohorite, minrecordite, benstonite, olekminskite, nyerereite, natrofairchildite, farichildite, zemkorite, butschlite, shrtite, remondite, petersenite, calcioburbankite, burbankite, khanneshite, carboncernaite, brinkite, pryrauite, and strontio, dressenite and other such materials or any combinations of these. The reforming catalyst may be any suitable hydrocarbon reforming catalyst, but preferably, the reforming catalyst metal component is selected from nickel, platinum, rhodium, palladium, ruthenium, or any effective combination of these. One of skill in the art should know and appreciate that the reforming catalyst metal is preferably supported on a high surface area, inert support material. Such supports may be selected from alumina, titania, zirconia, or similar such materials or combinations of these. The temperature of the reacting step should be maintained in a range that is sufficient to support the reforming reaction and to achieve the desired outcome of producing a hydrogen rich gas. In one preferred and illustrative embodiment, the temperature of the reacting step is maintained in a range from about 400xc2x0 C. to about 800xc2x0 C., more preferably a temperature range of about 450xc2x0 C. to about 700xc2x0 C. is used and especially preferred is a temperature for the reacting step from about 500xc2x0 C. to about 650xc2x0 C. The illustrative method is carried out such that the hydrogen rich gas is suitable for use in a fuel cell and more preferably has a carbon monoxide concentration less than about 10 wppm.
The present disclosure also encompasses a method for operating a fuel cell. Such an illustrative and preferred method includes: reacting a hydrocarbon fuel with steam in the presence of reforming catalyst and carbon dioxide fixing material to produce a first hydrogen gas; and removing carbon monoxide from the first hydrogen gas to produce a hydrogen rich gas. The removing of carbon monoxide step preferably utilizes a process for substantially decreasing the content of the carbon monoxide present in the hydrogen containing gas such as methanation or selective oxidation. Once generated, the hydrogen rich gas is fed to the anode of the fuel cell, in which the fuel cell consumes a portion of the hydrogen rich gas and produces electricity, an anode tail gas, and a cathode tail gas. The illustrative method may further include feeding the anode tail gas and the cathode tail gas to an anode tail gas oxidizer to produce an exhaust gas. As an alternative the cathode tail gas may be substituted by another oxygen gas source and combined with the anode tail gas and combusted to achieve substantially the same results. The exhaust gas so generated may subsequently be used to regenerate the carbon dioxide fixing material.
Further integration of the process is contemplated such that the method may include preheating process water with the anode tail gas and the cathode tail gas, such that the preheated process water is used to regenerate the carbon dioxide fixing material. The carbon dioxide fixing material may be selected from any suitable material that substantially decreases the content of the carbon dioxide in the hydrogen containing gas. Preferably, the carbon dioxide fixing material is selected from calcium oxide, calcium hydroxide, strontium oxide, strontium hydroxide, or similar mineral materials such as allanite, andralite, ankerite, anorthite, aragoniter, calcite, dolomite, clinozoisite, huntite, hydrotalcite, lawsonite, meionite, strontianite, vaterite, jutnohorite, minrecordite, benstonite, olekminskite, nyerereite, natrofairchildite, farichildite, zemkorite, butschlite, shrtite, remondite, petersenite, calcioburbankite, burbankite, khanneshite, carboncernaite, brinkite, pryrauite, strontio and dressenite and such materials or any combination of these. The temperature of the reacting step should be maintained in a range that is sufficient to support the reforming reaction and to achieve the desired outcome of producing a hydrogen rich gas. In one preferred and illustrative embodiment, the temperature of the reacting step is maintained in a range from about 400xc2x0 C. to about 800xc2x0 C., more preferably a temperature range of about 450xc2x0 C. to about 700xc2x0 C. is used and especially preferred is a temperature for the reacting step from about 500xc2x0 C. to about 650xc2x0 C. The illustrative method is carried out such that the hydrogen rich gas is suitable for use in a fuel cell and more preferably has a carbon monoxide concentration less than about 10 wppm.
Other illustrative methods of the present invention include: a method for operating a fuel cell, including: reacting the hydrocarbon fuel with steam in the presence of reforming catalyst and a material selected from calcium oxide, calcium hydroxide, strontium oxide, or strontium hydroxide to produce a first hydrogen gas, wherein the reaction temperature is from about 500xc2x0 C. to about 650xc2x0 C.; methanating the first hydrogen gas to produce a hydrogen rich gas having a carbon monoxide concentration less than about 10 wppm; feeding the hydrogen rich gas to the anode of the fuel cell, wherein the fuel cell consumes a portion of the hydrogen rich gas and produces electricity, an anode tail gas, and a cathode tail gas; and feeding the anode tail gas and the cathode tail gas to an anode tail gas oxidizer to produce an exhaust gas.
Another encompassed method includes a method for operating a fuel cell, including: reacting the hydrocarbon fuel with steam in the presence of reforming catalyst and a material selected from calcium oxide, calcium hydroxide, strontium oxide, or strontium hydroxide to produce a first hydrogen gas, wherein the reaction temperature is from about 500xc2x0 C. to about 650xc2x0 C.; methanating the first hydrogen gas to produce a hydrogen rich gas having a carbon monoxide concentration less than about 10 wppm; feeding the hydrogen rich gas to the anode of the fuel cell, wherein the fuel cell consumes a portion of the hydrogen rich gas and produces electricity, an anode tail gas, and a cathode tail gas; and preheating process water with the anode tail gas and the cathode tail gas, wherein the preheated process water is used to regenerate the carbon dioxide fixing material.
The present disclosure also encompasses an apparatus for producing electricity from hydrocarbon fuel, that substantially carries out one or more of the methods disclosed herein. In one such illustrative embodiment, the apparatus includes: at least two reforming catalyst beds, in which each reforming catalyst bed is composed of a reforming catalyst and carbon dioxide fixing material; a first manifold that is capable of diverting a feed stream between the at least two reforming catalyst beds; a reactor that is capable of producing a hydrogen rich gas by reducing the carbon monoxide concentration of the effluent of at least one of the reforming catalyst beds; and a second manifold that is capable of diverting the effluent of each reforming catalyst bed effluent between the reactor and exhaust. In one preferable and illustrative embodiment, the reactor is designed such that the level of carbon monoxide in the hydrogen containing gas is selectively and substantially decreased and more preferably is a methanation reactor or a selective oxidation reactor. The illustrative apparatus further includes a fuel cell that produces electricity and converts the hydrogen rich gas to anode tail gas and cathode tail gas. Another illustrative apparatus includes a metal hydride storage system that stores the hydrogen rich gas for use at a latter time. Yet another illustrative embodiment includes an anode tail gas oxidizer that combusts the anode tail gas and cathode tail gas to produce an exhaust gas. A third manifold can also be included in the illustrative apparatus disclosed herein that is capable of diverting the exhaust gas to at least one of the reforming catalyst beds for regeneration. The illustrative apparatus can be designed such that a water preheater is included, in which the water preheater heats process water using the anode tail gas and the cathode tail gas. Alternatively, the first manifold can be designed such that the first manifold is capable of diverting the preheated water to at least one of the reforming catalyst beds for regeneration.